Covering materials for agricultural houses and the like are required to be excellent in transparency and to have no changes in strength and transmittance of visible light even in outdoor exposure over a period of 10 years or longer. Therefore, fluororesin films represented by ETFE (ethylene-tetrafluoroethylene copolymer) have been widely used.
In agricultural house cultivation, scattering properties of transmitted light through the film of the agricultural house have influences on improvements in color, sugar content and yield depending on the kind of fruit, flower or vegetable, or on prevention of disease occurrence. That is to say, when, among the light transmitted through the film, much light is directly transmitted, there has sometimes occurred leaf scorch caused by direct sunlight or seedling blight due to a rapid increase of the internal temperature of the house. Further, because of decreased scattered light, shadow areas by crops or frames such as supports of the house increases. Accordingly, crops in the shadow areas show poor growth, give irregularity in color, or show a growth rate deviated from that of the crops in sunny areas. It has therefore become difficult to control cultivation and harvest times.
Methods for solving these problems include a method of allowing light transmitted through the film as the covering material to be scattered. Patent Document 1 shows a method of dispersing composite particles comprising silica, talc, and mica into a fluororesin to allow light to be scattered. Further, Patent Document 2 shows a method of providing a concavo-convex rough surface on at least one surface of a film to allow light to be scattered.